In many optical systems, such as telecommunications networks, optical sensors, etc., proper operation depends upon precise registration of an optical signal to a specific wavelength of light. In a wavelength-division-multiplexed (WDM) optical telecommunications system, for instance, information is carried on multiple optical signals, each of which characterized by a different wavelength of light. For proper system operation, the wavelength of each optical signal must be characterized by the correct wavelength. When one or more of the wavelength signals drifts from its defined wavelength, due to temperature or alignment changes, the signal-to-noise ratio (SNR) of the information carried on these signals is degraded. As a result, tremendous effort is expended to ensure proper registration of the WDM signals to the standardized International Telegraph Union (ITU) wavelength grid.
In some optical sensor systems, a change in the wavelength of one or more spectral features in an output signal indicates a change in a parameter being sensed. Proper system operation depends on an accurate knowledge of the absolute wavelength values of these spectral features.
Often, proper wavelength registration of a laser source in such systems is maintained by a feedback system comprising a calibrated gas cell, where the output wavelength of the laser-based transmitter is adjusted based on one or more spectral signals provided by the gas cell. A gas cell is an optical element that contains a known gas that absorbs a characteristic set of specific wavelengths (i.e., its absorption spectrum). Because the absorption spectrum of a gas is not a function of temperature, a gas cell provides a temperature-invariant wavelength reference to which the output of the laser source can be compared. In typical operation, the output of the gas cell is detected at a photodiode, amplified, and digitized. The digitized signal is then processed in a microprocessor, which, in turn, controls the temperature or electrical bias of the laser to tune its output wavelength to its proper wavelength.
Unfortunately, in many systems, even the use of feedback control does not guarantee that the optical signal of concern is held to its desired wavelength with sufficient precision to avoid optical and/or electrical errors in system operation. An ability to avoiding the transmission of erroneous output data from such systems would be a significant advance in the state of the art.